When coke is produced in a coke oven, it is progressively removed in batches one after another from a battery of retorts. Each retort yields a large incandescent mass that is pushed from the retort at a temperature of the order of 2000.degree.F. Being a combustible material comprised principally of carbon, it will readily burn if exposed to the air. Consequently, it must be protected from burning and cooled below an ignition temperature.
Generally, this has been done by quenching it with large quantities of water with the resulting steam being removed as saturated steam, quenching taking place of course from the outside toward the center of the mass. Water is a highly effective coolant, both because of its considerable specific heat but, more importantly, because of the large amount of latent heat, or heat of vaporization, which is required to convert water from a liquid to a gaseous state. However, contacting the incandescent coke with quantities of water results in the conversion of water to steam with explosive rapidity, resulting in fragmentation of the coke and the production of an undesirable quantity of fines. Both the steam and the fines give rise to pollution problems of such magnitude that the problem of protecting the surrounding air imposes tremendous expense.
Other processes have been perfected for the continuous cooling of coke wherein successive charges are discharged into the top of shaft type cooling units through which inert gas is circulated from the lower end toward the top of the cooler. This inert gas is removed from the upper end of the shaft at high temperature and circulated through a waste heat boiler to generate steam and partially cool the gases, which, however, may then require further cooling in a heat exchange unit of some type to be effectively cooler than the coke in the lower portion of the column. Thereafter, the cooled gases are recirculated to the shaft cooler.
This process requires that the coke be cooled generally to a temperature of around 400.degree.F, that is below a temperature where the coke will burn upon being discharged from the cooler into the atmosphere. The disadvantage of this method, however, is that the cooler the coke becomes, the lower the temperature of the inert gas must be in order to effectively cool it, and, even then, large volumes of inert gas are required to be circulated, adding both to initial plant cost and to subsequent operation.
Attempts to continuously cool with water involve more expensive and different procedures. It is obvious that an attempt to use steam in place of inert gas in a shaft cooler would result in the generation of water gas or producer gas because superheated steam in contact with incandescent carbon in an enclosure results in the dissocation of H.sub.2 O, resulting then in CO + H.sub.2. Hence, after the specific heat and the latent heat cooling effect of water have been used, the steam, unlike inert gas, cannot be used to remove more heat.
According to the present invention, coke is continuously cooled in a shaft cooler where the temperature differential between an inert gas and the coke results in a rapid removal of heat, but, as the coke reaches a temperature of 600.degree.F to 800.degree.F, it is discharged from the lower end of the shaft. It leaves the lower end of the shaft and moves through a chute to a quenching bin, both enclosed. As the coke moves down the chute to the bin, it is sprayed with water. At this lower temperature a relatively small volume of water at perhaps tap water temperature, or even warmer, requiring considerable heat to raise it to the boiling point and its high latent heat factor, or heat of vaporization, somewhere over 900 B.T.U. per pound, will cool the 600.degree. or 800.degree. coke below its ignition temperature. Moreover, the quenching will be far less violent.
An important incidental advantage is that the inert gas need not be cooled to nearly as low a temperature to be effectively recirculated and the volume of inert gas will be reduced.
With this combination, inert gas is used in the area of cooling the coke where it is most advantageous, i.e., where the temperature differentials are the greatest and convective cooling is the most effective while water is used in the range where its cooling capacity, depending as it does primarily on the transfer of heat energy as latent heat, is greatest and the least amount of water is required.
To assure that the coke will be sufficiently cool to be discharged from the quenching bin to the conveyor on which it is carried to a point of storage, more water may be sprayed on it in the quenching bin, this being preferably so regulated that the coke leaving the bin will even feel damp to the touch.
It is, of course, important that the application of water to the coke be effected after its removal from the bottom of the shaft in order to assure that no steam from the quenching will enter the shaft where, mixed with the inert gas, it would react with the high temperature coke, as above described.